Sync stretcher



May21,1957 H. R. sMlTH Erm. 2,793,289

' smc s'rRETcHER Y Filed nay 2, A1952 .s sheets-sheet 1 fier-1- ATTORNEY May 21, 1957 H. R. SMITH ET AL SYNC STRETCHER s sheets-sheet 2 Filed May 2, 1952.

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SYNC STRETCHER Filed May 2, V1952 3 Sheets-Sheet 3 fir-5- 70 No Daz/970A L: 6,9/0:

ATTORNEY the. signal to zero potential;

United States Patent* SYNC STRETCHER Harry R. Smith, Verona, and Allen- R. Taylor, Belleville, N. J., assignors to Standard Electronics Corporation,

Newark, N. J., a corporation of Delaware Application May 2,1952, Serial No. 285,768

3 Claims. (Cl. Z50-27)' trol in the amplifier which feeds the modulator grids of a television transmitter. The controlV essentially comprises an amplifier stage which includes a pair of amplifier tubes to which the composite video signalV is `simultaneously applied. The first tube of t-he pair substantiallyv linearly amplifes the entire composite signal; i. e. the synchronizing pulses and the vide'o portion alike. The second amplifier tube of the pair is biasedl substantially to cut-off during the application of the video portion of the composite signal thereto, but becomes: conductingwhen the synchronizing pulses are applied thereto. The character of the composite signal is such that' the synchronizing pulses are at: the maximum potential". Thi-s is accomplished by selecting as' the synchronizingpulsestretcher a stage in the multistage video amplifier wherein the polarity of the composite signal is such that they to vary the amount of bias on the amplifier tube whichY is normally biased to cut-ofi?. Accordingly, the non-linear amplifier stage may be termed a variable sync stretcher.

The following is a detailed' description of the invention in conjunction with a drawing, wherein:

Fig. l shows the sync stretcher stage of a video amplifier according to this invention;

Fig. 2 shows a seriesofsuitablepreamplifier stages for u-se in present invention; and,

Fig. 3 shows a suitable-finalvideo amplifier' stage for use in fthe present invention.

The coupling condenser for the input of the sync stretcher stage is shown at 1 in- Fig. l-. The D: C. restorer tube -is diode 2` Ihavingv its plate connected to the input circuit through resistor 3. The combination of condenser 1, diode 2 and resistor' 3 Will be recognized as a negative clamping circuit which operates in the manner well known to limit the most positive excursions of A-sf stated; above; the: number of amplification stages ahead of the. sync stretcher In this application a diode clamping circuit is'- ICC is such that the synchronizing pulses are maximum or uppermost commonly called black positive. The. func-y tion of the D. C. restorer may be briefly describedf by stating that thev condenser' 11 becomes charged to the maximum applied voltage and remains so charged' so #that :any swing necessarily is in the negative direction, and the output voltage varies between zero and some negative value, depending on the amplitude of the. input signal. The clampedk signal is applied through resistor 4 to the control grid of amplifier tube 5. The platel circuit of tube 5 includes resistors 6 and 7 and inductancey 8. The cathode is connected to ground throughunbypassed resistor 9v and toa metering circuit throughv r'e.- sistor 10. The filament' supply is filtered at radio 'fre'- quencies by condenser 11.

At 14 is the-usual gridy resistor to provide a D. C. path for the grids of 5 andf 12 to ground. At 4, 13,A 6 and7 15 are resistors of relatively low valve which perform' the usual function a-sl parasitic Suppressors (i. e. prevent'. unwanted oscillations). The plate of tube 12 is con'- nected to thejunctionbetween plate resistors 6 and'7 of tube Sthrough resistor 1-5. An adjustable bias sourcey for tube 12 is provided in resistors 16 and 18 and variable resistor 17 which form a potential divider betweenl the' 300 source of By supply and ground. The bias on" thecathode of tube 12 can be adjusted by resistor 17'. A cathode bypass condenser'19 and a filament supply filter' condenser 20 are -alsoprovided. The output of the'syncl stretcher sta-ge is taken from the junction of plate re'- -sistors 6, 7Y and 15r andfed` through condenser 21 to the input of the final amplifier stage shown in Fig. 3.

The operation4 of the circuit of Fig. lris as follows; The negatively clampedcomposite signal is simultane ously applied to the inputs. of tubes 5 and 12. Tube' 5'- is a class A amplifiery and: linearly amplifies the entirel signal. During the application of all but the synchronizing pulses of the videoy signal, the expander tube 1'2I isf cut off by the bias applied thereto through 16, 17 and 18. When, however, the synchronizing pulses are applied, beingat. the most maximum potential, the cutr off` bias is overcome and plate current flows in tube 1-2.. Since resistor 7V and inductance 8 are output impedances` common to both of tubes 5 and 12 it will be seen that. the plate currents of both tubes combine therethrough. during the application of synchronizing pulses. The re sult is that the synchronizing pulses are amplified more than the restof the composite signal. Moreover,V the amount of such relative amplification or sync stretching, may be controlled by varying resistor 17. Three stages of amplificationv in the form of tubes 22, 23 and 24 are provided ahead of the sync stretcher stage 'to give* theI desired signal amplitude and polarity as described above. The final amplifier stage includes an additional amplifier tube 25 preceded by a D. C. restorer in the form of az positive clamping circuit in diode 26 connected so that. the signal isapplied to the cathode thereof.

The plate of clamping diode 2 is connected to the inputs of amplifier-s 5 and 1'2'- by direct current conducting means which include resistors 4 and 13. This insures that the synchronizing signal component supplied by am-l plifier 12 and added to the linearly amplified composite signal. supplied by amplifier 5 in #the common output impedance 7 and 8 will be independent of, the picturecontent, or average brightness of the composite signal` The" amount of sync stretching accomplished by the am# plificr is therefore dependent solely uponthe settingV of the expander control 17.

Although the invention is, of course, not lin-iit'ed to the specific circuitV shown, nor to particularvaluesf of circuit components, the following. values have: been fou:n'dto1pro'^A ducef a satisfactoryI videov amplifier in# accordance with this invention:

3 Resistors R1D 220 ohms RZD 250 ohms R3D 100 ohms RSD 5,000 ohms, l W RGD 820 ohms, 2 W (15%) R7D 1,000,000 ohms R8D A 51 ohms, 2 W R9D 10,000 ohms R10D 1,000 ohms, 5 W R11D 10,000 ohms RlZD 50,000 ohms, 2 W R13D 100 ohms, 2W R14D 10,000 ohms R15D 50,000 ohms, 10 W R16D 1,000,000 ohms R17D 220 ohms, 2 W R18D 10,000 ohms R19D 900 ohms, 5 W R20D 47 ohms, 1 W R21D 2,000 ohms (il%) R22D 10 ohms R23D 1,000 ohms, 10 W R24D 150 ohms, 5 W RZSD 39,000 ohms (i 5%) R26D 10 ohms R27D 50,000 ohms, 4 W R28D 1,000 ohms R29D 1,000,000 ohms R30D 30,000 ohms, 3 W R31D 1,000,000 ohms RSZD 250 ohms, 10 W R33D 100,000 ohms, 1 W (I 5%) R34.1D 2,500 ohms, 10 W R34.2D 2,500 ohms, 10 W R41D 100 ohms, l W R42D 10,000 ohms R43D 10,000 ohms R44D 4,700 ohms R4SD 20,000 ohms, 5 W

Condensors C1D .5 mfd., 600 v. C2D 10 mfd., 600 v. CBD .5 mfd., 600 v. C4D .5 mfd., 600 v. CSD .5 mfd., 600 v.l C6D 1.0 mfd., 600 v. C7D 1,000 mfd., 50 v. CSD 2.0 mfd., 600 v. C10D 1.0 mfd., 600 v. C11 .001 mfd. C12 .001 mfd. C13D 80 mfd., 450 v. C14 .001 mfd.

C16 .001 rnfd. C17 .001 mfd C18 .001 mfd. C19 .001 mfd.

Tubes VID 6AG7 VZD 6AG7 VSD 6AG7 V4D 6AL5 VSD 6AG7 V6D 6BG6 V7D 6BG6 VSD 1/26AL5 We claim:

1. A video amplifier compri-sing in combination a direct current restorer which consists of a negative clamping circuit to which an unclamped composite signal is applied, said composite signal comprising a video portion,

the positive potentials of which correspond to black picture content and synchronizing pulses, said synchronizing pulses being at a higher potential than said video portion, said negative clamping circuit having a rectifying element so poled as to limit the most positive excursions of the composite signal substantially to zero potential, first and second amplifier tubes each having a cathode, a control grid, and a plate, direct current conducting means con= ducting the composite signal proceeding from said negative clamping circuit to the control grid of said first amplifier tube, which substantially linearly amplifies the same, and also to the control grid of said second amplifier tube, circuit and potential means to operate said first amplifier tube substantially as a class A amplifier, direct current conducting means applying a bias between the cathode and control grid of said second amplifier tube which substantially cuts off plate current flow therethrough during the application of all of said composite signal thereto except said synchronizing pulses, the plates of said first and second amplifier tubes being connected to an output circuit such that the amplification of said video amplifier increases during the application of said synchronizing pulses.

2. A video amplifier comprising in combination first and second amplifier tubes each having a cathode, a conw trol grid and a plate,` a negative clamping circuit comprising a diode having aplate and a cathode across which the input to said video amplifier is applied, direct current conducting means connecting the plate of said diode fto the control grid of each of said first and second amplier tubes, adjustable bias means for said second amplifier tube biasing the control grid of the latter negative with respect to the cathode thereof by an amount sufficient substantially to cut off plate current flow therethrough for input signals below a predetermined amplitude appearing on the plate of said diode, the plates of said first and second amplifier tubes being connected to a source of direct current potential through a common load im'- pedance.

3. A video amplifier comprising in combination first and second amplifier tubes each having a cathode, a control grid and a plate, a negative clamping circuit across which the input to said video amplifier is applied, said negative clamping circuit comprising a rectifying'element so poled as to limit positive excursions of the signal proceeding therefrom substantially to zero potential, direct current conducting means connecting said negative clamping circuit to -the control grid of each of said first and second amplifier tubes, adjustable bias means for said second amplifier tube biasing the control grid of the latter negative With respect to the cathode thereof by an amount sufficient substantially to cut off plate current fiow therethrough for input signals below a predetermined amplitude applied across said negative clamping circuit, thc plates of said first and second amplifier tubes being connected to a source of direct current potential through a common load impedance.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Elements of Television Systems, by George E. Anner, published by Prentice-Hall, Inc., 1951, page 510. 

